Aspects on ventilation induced stress and strain on regional and global inflammation in experimental acute respiratory distress syndrome

Retamal Montes, J. 2016. Aspects on ventilation induced stress and strain on regional and global inflammation in experimental acute respiratory distress syndrome. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 1235. 64 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9612-8. Mechanical ventilation (MV) is a life-saving therapy in acute respiratory distress syndrome (ARDS), a condition that affects 3000 patients/year in Sweden with a mortality rate of about 40%. However, MV may induce or worsen lung injury causing “ventilator-induced lung injury (VILI)”. From a mechanical perspective strain (deformation, or relative change in lung volume) and stress (tension) have been postulated as main determinants of VILI. High respiratory rate is potentially another factor that may exacerbate VILI by amplifying the total energy transmitted to the lungs during MV. In this thesis in animal ARDS models the hypotheses were that 1) lung parenchyma inhomogeneities concentrate stress and amplify lung damage and inflammation, 2) higher respiratory rates increase lung inflammation and lung edema in heterogeneous ARDS, and 3) local lung deformation is related to local inflammation. First, in a rat model the effect on inflammation and structural damage of regional lung collapse on the healthy surrounding lung tissue was assessed. Second, in porcine models the effect of respiratory rate on lung edema and inflammation was studied during two ventilatory modes; a) a permissive collapse mode and b) a homogenized lung parenchyma mode. Finally, lung deformation was correlated with lung inflammation assessed by positron emission tomography using 18F-FDG uptake. It was found that; 1) local inhomogeneities can act as stress amplifiers, increasing lung tissue inflammation and damage in the healthy surrounded lung. 2) high respiratory rate increases lung edema but decreases lung inflammation when permissive lung collapse is used and that these effects are prevented with lung parenchyma homogenization; 3) local lung deformation and inflammation are well correlated. In conclusion, lung inhomogeneities may aggravate VILI, respiratory rate may affect in different ways VILI progression depending on the ventilatory strategy, and finally, lung deformation is closely related to lung inflammation. With the caveat that the studies are performed in animal models, the results suggest that using ventilator strategies that homogenize the lungs, i.e., open collapsed lung regions and prevent re-collapse in ARDS will reduce VILI and in the end may decrease morbidity and the high mortality in this condition.